Field of the Invention
The present invention is directed to an article lifting device of the type employed to lift or lower articles in a production line setting, typically in an application where the lifting device may be mounted upon a horizontally extending shuttle unit in which the lifting device lifts an article from one work station, is transferred horizontally by the shuttle unit, and then lowers the article at a second work station.
Article lifting devices of the general type with which the present invention is concerned are disclosed in U.S. Pat. Nos. 4,475,863 and 4,444,540. The devices disclosed in those patents include a vertically elongate hollow tubular lifting member which may be mounted upon a shuttle unit, and a second vertically elongate hollow tubular member telescopically received within the first tubular member for vertical reciprocation within the first tubular member. An article holding device is mounted on the bottom of the lifting member, and power driven means are employed to drive the lifting member in vertical movement within first member. In the case of U.S. Pat. Nos. 4,444,540 and 4,475,863, referred to above, the power drive takes the form of a vertically extending ball screw mounted on the fixed member operatively received within a nut on the second or lifting member, the ball screw being driven by a reversible motor. While this type of drive, particularly when operated with an electronic control, can achieve an extremely precise positioning of the workpiece, the ball screw and nut coupling between the relatively movable tubular members requires a substantial amount of clearance, and the speed of the drive, particularly where relatively heavy workpieces are handled, has some practical limitations.
In other arrangements, the drive takes the form of a relatively long stroke pneumatic motor having its cylinder coupled typically to the outer or fixed frame member and its piston rod to the vertically movable lift member. This arrangement also presents clearance problems and control problems. Where a relatively heavy part is coupled to the lift member, pressure must build up beneath the piston until the weight of the article is overcome. If air under pressure is admitted rapidly, the lifting force developed by the piston likewise increases rapidly with the result that the workpiece is subjected to a high initial acceleration, rather than being smoothly lifted from dead rest. A similar problem is encountered at the opposite end of the stroke. Further, practical limitations on the weight of workpieces which can be handled arise in that clearance problems impose limitations on the cylinder diameter.
The present invention is directed to an article lifting device employing a tubular lifting member telescopically received within a fixed or outer tubular member which is provided with a drive located externally of the lifting member to minimize clearance problems, which may be smoothly accelerated and decelerated and precisely positioned, and which is capable of handling workpieces of substantial weight.